Effect of detached/re-suspended solids from sewer sediment on the sewage phase bacterial activity

This study was conducted to make an assessment of the effects of the detached/re-suspended solids on the bacterial activity in the sewage phase of a gravity sewer. A physical sewer-model was used to simulate two extreme conditions: sewage flow without the presence of sewer sediment, and filtered sewage flow with the sediment. The first scenario was to evaluate the effect of the settling of solids on the bacterial activity, while the second scenario was to examine the effect of purely re-suspended or detached solids. The water media were aerated to obtain an initial DO level at about 6 mg/L, and the bacterial activity was monitored at a regular time interval during each operation. Two bacterial cell staining techniques, one using 4′, 6-diamidino-2-phenyl indole (DAPI), and another using 5-cyano 2, 3-ditoyl tetrazolium chloride (CTC), were employed to measure the amount of total and respiring bacteria, respectively. Both the DAPI and CTC counts decreased with time in the first case, while that increased with time in the second case. The bacterial activity in a sewer phase was observed to be contributed by smaller sized particles. Also, the solids originated from the sewer sediment through re-suspension or detachment demonstrated a higher bacterial activity than the solids originally present in the sewage.